Infrared detector cell



@ 2mm@ @Ross REFERENCE .SEARCH J Oct. J. R. vJNNESS, JR l n n z ,n INFRARED DETECTOR CELL .A V

Filed April '19"1954-gm fitates @arent 2,758,255 aertted- Oct. 23, 1956 2,768,265 maken DETECToR CELL James R. Ienness, Ir., Southampton, Pa. i Application April 19, 1954, serial No. 424,288 11 (ci. zot-63 (Granted under Title 35, U. S. Code (1952), sec. 266) The invention described herein may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor.

This invention relates to a detector cell for infrared radiation, more particularly, it relatesto photooonductive infrared detectiagnngelmlsgnarglwjhe process f'f'linfnem. l'minfrared detection cells utilizing photoconductive coatings an appreciable amount of radiation is lost throughreection at the surface of the photoconductive coating, thus rendering it highly difficult to detect radiation of low intensity. Excessive reflection is due to the high index of refraction of photoconductive materials. The problem is further magnified by the fact that in prior detection cells it has been necessary to enclose the cell in an evacuated envelope to prevent exposure of the photoconductive coating, the envelope presenting a further Aproblem in absorption and reflection of incident radiation. It is an object of this invention to provide a means for increasing the eiciency of -photoconductive infrared detection cells.

It is another object of this invention to provide a means for increasing the efiiciency of a photoconductive infrared detection cell which is also effective to protect the photoconductive material from exposure to the air.

It is still another object of 'this invention to provide a means for decreasing the retiection of incident infrared radiation from a photoco'nductive surface.

It has been found that the Aabove and other objects are accomplished by applying to the photoconductive coating of the infrared detection cell a retiection reducing coating having an optical thickness for the radiation of principal interest which is an odd multiple of one fourth the wave length of the infrared radiation of principal interest and consisting of a material having an index of refraction for the infrared radiation of principal interest less than that of the photoconductive material. The efficiency of the cell is further increased by use of a basetransparent to infrared radiation which is coated with a retiecting coating on the .surface opposite the photoconductive coating, the

reflecting surface adapted to reect radiation which has passed through the photoconductive coating back through the coating a second time.

' will proceed through the base and be reflected -back surface is used. The coating 1'1 is of a photoconductive material such as lead sulfide, 'lead teluride or lead selenide. It is approximately from 1 to 2 microns in thickness, the

l.thickness being modified in the drawing, as in the case of The invention is best explained by reference to the accompanying drawing hereby made a part of this application'and in which the single figure is a vertical cross section of the infrared detection cell of-this invention. In the figure the numeral 10 represents the base of the cell. A'photoconductive coating 11 is applied to the base on one side as shown and aY reflecting coating 12 is applied to the opposite surface. A retiection reducing coating 13 is applied to the photoconductive coating and the photoconductive coating 11 is provided with electrical leads 14.

The base 10 is preferably made of a dielectric material such as sapphire, calcitun iiuoride or magnesium oxide which is transparent to infrared radiation. This is necessasy when the mirror coating 12 is used so that incident radiation passing through the photoconductve coating other coatings, for the purposes of illustration. It is ap Other means of applica plied by vacuum evaporation. tion may be used when suitable, such as, spraying chemical deposition' or others. "The mirror coating 12r may 'of conventional material used for this purpose, lsuch assilver or aluminum. It is applied by conventional techniques such as vacuum evaporation in the case of aluminum and chemical deposition or vacuum evaporation in the case of silver. The surface of the coating facingthe base 10 functions tovretlect radiation back through the hase and into the photoconductive. coating again. The reflection reducing coating 13 is restricted to a material having a number of we ll defined properties. To minimize the reection of incident radiation itsindcx of refraction should be lower than that of the phuoconductive coating, limit.c of 1.3 to 2.5 being preferable'. The index of refraction of lead sulfide, for example, is 3.9 for visible light and is believed tobe about 3.5 for infrared radiation ata wavelength of 2 microns. The ideal material for a -I y reflection reducing coating shouid have a refractive index, for a wavelength of 2 microns of n=\/3.5 or 1.85. However, since Ilead sulde reflects approximately 35% of the Aincident radiation, any material with an index of refraction less than that of lead sulfide, veven though its be by evaporation, sputtering, chemical deposition, spray# ing or by other conventional means. The material is applied preferably in a thickness which gives an optical thickness for the infrared radiation of principal interest which is an odd multiple of one-fourth the wavelength of vinfrared radiation of principal interest, this being the thickness prescribed by well known optical theory for ideal results. t

The actual thickness of the reflection reducing coating is computed by dividing the desired optical thickness by the index of refraction for the infrared wave length 0f principal interest in the material of the coating. A list of actual coating thicknesses for various materials is shown in the table below. The infrared wavelength radiation of principal interest is taken as 2 microns and i The vsymbol )t is used to denote the multiple as one.

wavelength.

. Rsfraetion i index for Optical Mechanical Coating Material Infrar Thickness Thickness Radiation for A=2 in Micron:

)tra Miernns l Micron:

Micron: 1. as s .a1 f 1. 38 5 L 6 l 6 33 3. 5 17 l. 8 5 2 42 5 206 L 8 5 2B By radiation of principal interest as used herein andY avancee red radiation of wave lengths vbetween about one and. rive terpreted to-include one as an odd multiple.

In operation the cell i's included in the circuit of a signal `detection devicei and mounted in the path of the target radiation. Radiation striking .the reflection re duction coating 13, which is transparent to infrared lra` of the sulphur group, .and a reflection reducing coating microns. An odd multi-ple of one-fourth" is to be inl ldiation, passes into the photoconductive coating 11 where it is transformed into electrical energy which is detected as a signal. The coating 13 being of the prescribed material and thickness prevents the reection of an appre ciable amount of radiation which would be ordinarily reflected at the first surface of the photoconductive coat- 'ing because of its high index of refraction. As photoconductive materials are somewhat transparent to infrared radiation, some of the radiation striking the photoconductive coating passes on through the thin coating. The radiation passing through the photoconductive coating 11 passes through the transparent base 10 and strikes the reflecting surface 12 from which it is reflected back through the base and into the photoconductive coating 11..t o produce additional electrical energy. The result of the additive effect of the reflection reducing coating in preventing unwanted reection of incident radiation and the reflecting surface introducing more radiation into the` photoconductive coating 11 causes an overall increase lin the eiciency of the cell, permitting the detection of weaker target signals. In addition to the increased efficiency resulting from the application of a reection reduction coating, the coating serves the additional function of protecting the photocondu-ctive coating from exposure, thus eliminating the -cessity of enclosing the cell in an evacuated envelope.

While the invention has been illustrated by a specic i structure, the application of the invention may take the form of various structures, and the invention is by no means intended to be limited to the structure shown.

I t is thus seen that there has been provided an infrared detector cell of increased efficiency in which reflection .of incident radiation has been minimized and a protective coating has been furnished for the photoconductive coat- Obviously, many modifications and -variations of the prt-sent invention are possible in the light of the above teachings. It is, therefore, to be understood that within the scope of the appended claims the invention may be practiced 'otherwise than as specifically described.

What is claimed is: l

l. In a photoopdptiveggllg in combination, a sup'C porting baseof'dilectrc material, a coatingppfha photo- Vflection edticing surface coatipgugf. diele tric material v orr'ductive 2. A photoconductive cell comprising a supporting base of dielectric material, a coating of photoconductive material on one side of said base, and a reflection re ducing surface coating on said photoconductive coating consisting of a material having an index of refraction for infrared rays less than that of said photoconductive material.

3. A photoconductive cell comprising a supporting Abase of dielectric material, a coating of photoconductive material on one side of said base, and a reection reducing coating on said photoconductive material having an optical thickness for the radiation of principal interest which is -an odd multiple of one-fourth of the wave length of radiation of principal interest and consisting of a material having an index of refraction for the radiation of principal `interest less than that of the photoconductive material. I v

4. A photoconductive cell for infrared radiation comprising a supporting base of dielectric material, a coating of photoconductive material on one side of said base consisting o f a binary compound of lead and a member having an index of refraction for the radiation ot pri'ncipal interestless than that of the photoconductive material for the radiation of 'principal interest.

5. A photoconductive cell comprising a supporting base of dielectric material, a coatingof photoconductive ma terial on one side of said base comprising a compound from the class consistingv of `lead Suliide, lead teluride and lead selenide, and a reflection reducing coating lon said photoconductive coating having an optical thickness for thel radiation of principal interest which is an odd multiple of one-fourth the wave length of radiation of principal interestr and consisting of a material having an index of refraction between about 1.3 and about 2.5 for the wave length of radiation of principal interest.

6. A photoconductive cell-for infrared radiation comprising a supporting base of a dielectric material, a coating Tf photoconductive material on one side of said base comprising a compound from the classV consisting of lead sulfide, lead selcnide and lead teluride, and a reflection f 7. A photoconductive cell for infrared radiation com prising a supporting base of dielectric material transpar ent to infrared radiation, 4a reflecting surface on one side'of said base facing said base and adapted to reflect ra.

diation through said base, a coating of -photoconductive material on the opposite side of said base, and a redection reducing surface coating on said photoconductivc coating.

8. A photoconductive cell for infrare prising a supporting base of dielectric material transparcnt to infrared radiation, a reflecting surface on one side of said base facing said base .and adapted to redect radiatio-n through said base, a coating of photoconductive material on the opposite side of said base, and a redection reducing coating on said photoconductive coating consisting of a material having an index .of refraction for the radiation of principal interest less than that of the said photoconductive material.

9. A photuconnuctive cell for infraredl radiation comV prising a supporting base of dielectric material transpar ent to infrared radiation, a redecting surface on one side of said base facing said base and adapted to rcfleet radiation through said base, a coating of photoconductive material on the opposite side of said base comprising a compound from the class consisting of lead sulfide, lead teluride and lead selenide, and a reiiection reducing coating on said photoconductive inaterial having an optical thickness for the radiation of principal interest which is an odd multiple of one-fourth of the wave length of radiation of principal interest and consisting of a material having an index of refraction forthe radiation of principle .interest between unity and Y thatAof the photoconductive material.

10. A photoconductive cell for infrared radiation oom-y prising a supporting base of dielectric material transpar ent to infrared radiation, a reflecting surface on one side of said base facing said base and adapted to reect radiation through said base, a coating of photoconductive material on the opposite side of said base comprising a compound from the class consisting of lead sulfide, lead teluride and lead selenide and a reilectionreducing coating on said photoconductve material having an optical thickness for the radiation of principal interest which is an odd multiple of one-fourth the wave length of I radiation of principal interest and consisting of a dielectric, solid compound transparent to infrared radiation.

d'radiationvcomt l1. A photoconductive-cell for infrared radiation, ecm-iprising, a supporting base of dielectric material transparnt :o infrared radiation, a reecting surface of alumi num on one side of said base operative to reect rada Lion through said base, a coating of lead sulphide on the opposite side of said base, and a coating of silicon monoxide on said lead sulphide coating having a of about .33 microns.

References Cited in tbe le of this patent l UNITED STATES PATENTSV Tonnies Nov. 22, 1938 Gibson ..--..--..--.l Mar. 6, i951 runner -.-f -4--- Apr. s, 195| thickness 

1. IN A PHOTOCONDUCTIVE CELL, IN COMBINATION, A SUPPORTING BASE OF DIELECTRIC MATERIAL, A COATING OF A PHOTOCONDUCTIVE MATERIAL ON ONE SIDE OF SAID BASE, AND A REFLECTION INDUCING SURFACE COATING OF DIELECTRIC MATERIAL TRANSPARENT TO INFRARED RADIATION ON SAID PHOTOCONDUCTIVE MATERIAL. 